1. Field of the Invention
The invention relates generally to electric motor control, and more specifically, to operating a clothes washing machine powered by an induction motor at a desired speed.
2. Description of Related Art
Residential and commercial clothes washing machines are well known. A generally cylindrical tub or basket for holding the clothing and other articles to be washed is rotatably mounted within a cabinet. Typically, an electric motor drives the basket. During a wash cycle, a cleaning solution of water and detergent or soap is forced through the clothes to wash them. The solution is rinsed from the clothes with water, then, during one or more spin cycles, the water is extracted from the clothes by spinning the basket.
One way of categorizing washing machines is by the orientation of the washing machine basket. A conventional vertical-axis washing machine includes a perforated wash basket arranged to spin about a generally vertical axis. The wash basket is disposed within a vertically aligned tub, and an agitator is situated in the wash basket. Articles to be washed are loaded into the basket through a door, which is usually situated on the top of the washing machine. The tub is filled with the detergent and water solution, and oscillation of the agitator moves the clothes in the solution. A motor typically drives the agitator, in addition to spinning the vertically-oriented basket during spin cycles. A series of gears or belts are commonly configured to drive the proper component at the proper time during each washing machine cycle.
In other vertical-axis washing machine configurations, articles to be washed are moved within the cleaning solution by means other than an agitator. For example, a system disclosed in U.S. Pat. No. 5,271,251 uses a series of inwardly directed baffles. In association with the baffles, ramps on the bottom wall of the basket cause the articles within the basket to move up the side of the basket wall and to slide along the baffles in a swirling motion as the basket is turned by the motor. In another vertical-axis washing machine that is described in U.S. Pat. No. 5,460,018, the agitator is eliminated by use of a xe2x80x9cwobblexe2x80x9d plate that is disposed within the lower portion of the wash basket and mounted for wobbling motion. The motor is selectively interconnected with the basket and bottom plate for rotating the basket and the bottom plate together and for wobbling the bottom plate relative to the wash basket while the basket is held stationary such that the clothes items are xe2x80x9cbouncedxe2x80x9d in and out of the cleaning solution within the wash basket.
Horizontal-axis washing machines, having the basket oriented to spin about an essentially horizontal axis, also do not include an agitator, and a variable-speed motor is typically used to drive the basket. During wash cycles, the basket of the horizontal-axis washing machines rotates at a relatively low speed. The rotation speed of the basket is such that clothes are lifted up out of the water, using baffles distributed about the basket, then dropped back into the water as the basket revolves. During a xe2x80x9cdistributionxe2x80x9d cycle, the horizontally-oriented basket rotation speed is gradually increased, until the clothes begin to xe2x80x9cstickxe2x80x9d to the sides of the basket due to centrifugal force, thereby distributing the clothes about the sides of the basket. Once the clothes have been distributed about the basket, the speed is further increased to extract the water from the clothes.
Driving the wash basket at predetermined speeds is necessary in many washing machine configurations. For instance, both vertical and horizontal-axis washing machines extract water from clothes by spinning the basket, such that centrifugal force extracts the water from the clothes. It is desirable to spin the basket at a high speed and extract the maximum amount of water from the clothes in the shortest possible time, thus saving time and energy. In vertical-axis machines without agitators, such as the wobble plate configuration, if the speed is too slow, the wash action may not be sufficient. Conversely, if the speed is too fast, the clothes may tangle. In horizontal-axis machines, if the basket is rotated too slowly, the clothes will not lift out of the water, and if the basket rotates too fast, the clothes will xe2x80x9cstickxe2x80x9d to the sides of the basket and not tumble into the cleaning solution. Thus, precise speed control is important for a washing machine to perform its function of getting clothes clean. Unfortunately, precise speed control mechanisms are often complicated and expensive.
Single phase AC induction motors are commonly used in washing machine applications. AC induction motors are popular for several reasons, including high robustness, reliability, low price and good efficiency. However, an induction motor""s speed slips relative to synchronous speed as the load increases. This could be unacceptable, since it may be important to maintain, or even increase, the wash basket speed as the load increases. For instance, it may be necessary to spin a large load of clothes faster than a small load to remove as much water as possible. In a vertical-axis machine employing a wobble plate to agitate the clothes, it may be necessary to drive the wobble plate faster for large loads to move the clothes in and out of the cleaning solution as desired. Similarly, in a horizontal-axis machine""s wash cycle, it may be necessary to rotate the basket faster with a large load so that the clothes lift up out of the cleaning solution.
For variable speed applications, three-phase AC induction motors are often used. They are typically powered by a three-phase pulse-width modulated (PWM) inverter. The desired speeds are achieved by setting the excitation frequency; however, the motor speed still slips relative to the excitation frequency as the load increases.
Expensive, nearly synchronous motors are generally inappropriate for the cost-sensitive appliance market. Tachometer feedback closed-loop speed control is used in some washing machine applications, although the tachometer hardware adds unwanted costs to the system. In other systems, the inverter output voltage is varied in response to increased loads to affect slip and maintain the desired speed. Varying inverter output voltage complicates system implementation, is constrained in speed increment by the output frequency, and risks pullout due to transient loads in low volts/hertz situations.
Hence, a need exists for improved speed control in induction motor-driven washing machines. However, implementing such controls is difficult in clothes washing machine applications, since, among other things, the load varies greatly due to varying amounts of clothes and water in the machine. The present invention addresses shortcomings associated with the prior art.
In one aspect of the present invention, a method of driving a clothes washing machine motor at a predetermined speed is presented. The washing machine motor is powered by a DC to AC inverter that includes a DC bus, and the method includes measuring the DC bus current and adjusting the inverter output frequency in response to the DC bus current measurement. In exemplary embodiments, measuring the DC bus current includes measuring the voltage drop across a resistor coupled to the DC bus. The frequency adjustment may be calculated by multiplying the DC bus current by a compensation factor to calculate a frequency adjustment value. Alternatively, frequency adjustment values may be stored in a look-up table, and the measured DC bus current is used to index the look-up table.
In accordance with other aspects of the present invention, a clothes washing machine motor control system includes a DC voltage source including a DC bus and an inverter including a plurality of switches coupled to the DC bus. The switches are arranged to selectively couple phase windings of a washing machine motor to the DC bus. A controller is coupled to the DC bus and to the inverter, with the controller being operable to drive the switches so as to provide pulse-width modulated sine wave voltages to the phase windings. The controller receives an indication of the DC bus current and, in response thereto, adjusts the inverter output frequency.
In accordance with further aspects of the present invention, a clothes washing machine system includes a cabinet, a tub rotatably mounted within the cabinet, and a motor including a plurality of phase windings therein. The motor is operably coupled to the tub for rotating the tub within the cabinet. A DC voltage source includes a DC bus, and an inverter that includes a plurality of switches is coupled to the DC bus, with the switches arranged to selectively couple the phase windings to the DC bus. A controller is coupled to the DC bus and to the inverter, with the controller being operable to drive the switches so as to provide pulse-width modulated sine wave voltages to the phase windings. The controller receives an indication of the DC bus current and in response thereto, adjusts the inverter output frequency. The washing machine may be either a vertical-axis machine, with the washing machine tub mounted within the cabinet so as to rotate about a generally vertical axis, or a horizontal-axis machine, with the washing machine tub mounted within the cabinet so as to rotate about a generally horizontal axis.
According to still further aspects of the invention, a program storage device includes instructions that when executed, perform a method of driving a washing machine motor at a desired speed. The washing machine motor is powered by a DC to AC inverter that includes a DC bus. The method includes receiving an indication of the DC bus current, and determining an adjustment to the inverter output frequency in response to the indication of the DC bus current. In specific embodiments, the voltage to frequency ratio of the inverter output is kept essentially constant.